1. Field of the Invention
The invention relates to a vaporized fuel purge system, and particularly, to a vaporized fuel purge system suitable for purging vaporized fuel generated within a fuel tank of a vehicle.
2. Description of the Related Art
Patent Application Publication No. H7-259615 discloses the device that adsorbs vaporized fuel generated in the fuel tank so as to be held in the canister, and purges the vaporized fuel into the intake passage during operation of the internal combustion engine. In order to constantly maintain the control accuracy of the air-fuel ratio in the aforementioned device, the fuel injection amount is required to be reduced during purging by the amount corresponding to that of the vaporized fuel to be supplied.
The aforementioned device includes an oxygen sensor disposed in the exhaust passage of the internal combustion engine to satisfy the aforementioned requirement. The oxygen sensor generates a signal indicating the exhaust air-fuel ratio being either rich or lean.
The device then calculates the feedback correction coefficient FAF for correcting the fuel injection amount based on the output of the oxygen sensor. Specifically, the fuel injection amount is increased as the FAF becomes larger, and is decreased as the FAF becomes smaller.
The feedback correction coefficient FAF is updated in an increasing direction while the output of the oxygen sensor indicates lean, and is updated in a decreasing direction while the output of the oxygen sensor indicates rich. If the exhaust air-fuel ratio is lean, the fuel injection amount is gradually increased toward a theoretical air-fuel ratio. If it is rich, the fuel injection amount is gradually decreased toward the theoretical air-fuel ratio.
Purge gas of the vaporized fuel supplied into the intake air passage influences the exhaust air-fuel ratio. For example, if the purge gas is fuel-rich, the exhaust air-fuel ratio will shift to rich. Then the feedback correction coefficient FAF is updated in the decreasing direction to prevent the shifting of the air-fuel ratio to rich. As such, the effect of the fuel-richness of the purge gas is absorbed by the FAF, whereby the exhaust air-fuel ratio is maintained around the theoretical air-fuel ratio.
In the aforementioned case, the feedback correction coefficient FAF becomes smaller than a reference value (for example, 1.0) by the value corresponding to the fuel-rich level of purge gas. Therefore, it is possible to estimate the fuel-rich level of the purge gas based on the difference between an actual value of the FAF and the reference value. Likewise, if the purge gas is fuel-lean, the feedback correction coefficient FAF becomes larger than the reference value by the value corresponding to the fuel-lean level of purge gas. It is also possible to estimate the fuel-lean level of the purge gas based on the difference between the actual value of the FAF and the reference value.
In the aforementioned device, the fuel concentration of purge gas is learned in order to reduce the fuel injection amount by the amount of the supplied vaporized fuel. Specifically, when the feedback correction coefficient FAF becomes larger than the reference value by 2% or more during purging of vaporized fuel, it is considered that the current fuel concentration is excessively low. The fuel concentration is then updated to rich by a specified update value. Meanwhile, when the feedback correction coefficient FAF becomes smaller than the reference value by 2% or more, it is considered that the current fuel concentration is excessively high. The fuel concentration, thus, is updated to lean by a specified update value.
In the aforementioned process, the fuel concentration of purge gas can be correctly learned. If the fuel injection amount is reduced based on the correctly learned fuel concentration, the influence of the vaporized fuel may be eliminated. Accordingly, the accuracy in controlling the air-fuel ratio may be maintained during purging of the vaporized fuel.
The fuel concentration of purge gas cannot be maintained constant during operation of the internal combustion engine. For example, when the amount of the fuel adsorbed in the canister has decreased to a certain level during purging of vaporized fuel, the fuel concentration of purge gas decreases sharply. Generally, the learned value of the fuel concentration of purge gas reflects changes in the fuel concentration of purge gas via the feedback correction coefficient FAF. However, if the fuel concentration changes sharply, the learned value is likely to largely deviate from the actual fuel concentration of purge gas.
In consideration of the above-described problem, the foregoing related-art device changes the update value of the fuel concentration of purge gas to be large in response to a sharp change in the fuel concentration of purge gas. Such a process makes it possible to update the learned value of the fuel concentration of purge gas so as to follow sharp changes in the actual fuel concentration. Accordingly, the device is capable of maintaining the accuracy of the air-fuel ratio control in spite of sharp changes in the fuel concentration of purge gas.
The aforementioned device is designed to update the fuel concentration of purge gas with predetermined update values. That is, the fuel concentration is always updated using the predetermined update values irrespective of how the learned value has been deviating from the actual fuel concentration of purge gas in each case.
This related-art device, therefore, may cause the problems to be described below. Firstly, when the learned value largely deviates from the actual fuel concentration of the purge gas, the fuel concentration is first updated, repeatedly, using a predetermined small update value. If a sharp change in the fuel concentration is then detected, the update value is changed to a predetermined large update value, with which the fuel concentration is further updated. In this case, the learned value of the fuel concentration may fail to timely approximate the actual fuel concentration.
Secondly, after detection of the sharp change in the fuel concentration, even if the deviation of the learned value from the actual fuel concentration becomes small, the learned value is continuously updated with the predetermined large update value until the determination of the sharp change in the fuel concentration is canceled. Thus, there may be the case that the learned value of the fuel concentration exceeds the actual fuel concentration, resulting in overshooting. Thus, the aforementioned related-art device has difficulties in learning the fuel concentration of purge gas timely and accurately.